For the past 150 years, lighting technology has been mainly limited to incandescence and fluorescence. While derivative technologies such as high-intensity discharge (HID) lamps have emerged, none have achieved energy efficiencies exceeding 25%, with incandescent lighting achieving an efficiency of less than 2%. With the advent of commercial light emitting diodes (LEDs) in the 1960s, however, the door was opened for a different and exciting form of lighting technology. Unlike conventional lighting, LEDs consume less electricity and have largely avoided the parasitic by-products of its predecessors, namely heat. Early LEDs were red in color, with yellow and orange variants following soon thereafter. To produce white light, however, a blue LED was needed. In 1993, Shuji Nakamura of Nichia Chemical Industries produced a blue LED using gallium nitride (GaN). With this development, it was now possible to create white light by combining the light of separate LEDs (red, green, and blue), or by creating white LEDs themselves by means of doping.
Solid state lighting (SSL) refers to a type of lighting that utilizes LEDs, organic light-emitting diodes (OLEDs), or polymer light-emitting diodes (PLEDs) as sources of illumination rather than electrical filaments or gas. Unlike traditional lighting, SSL creates visible light with very little heat or parasitic energy dissipation. Additionally, the solid-state nature provides for greater resistance to shock, vibration, and wear, thereby increasing lifespan significantly. SSL has been described by the United States Department of Energy as a pivotal emerging technology that promises to alter lighting in the future. It is the first new lighting technology to emerge in over 40 years and, with its energy efficiencies and cost savings, has the potential to be a very disruptive technology in the marketplace as well.
A single LED can produce only a limited amount of light, and only a single color at a time. To produce the white light necessary for SSL, light spanning the visible spectrum (red, green, and blue) must be generated in correct proportions. To achieve this effect, three approaches may be used for generating white light with LEDs: wavelength conversion, color mixing, and most recently homoepitaxial ZnSe.
Wavelength conversion involves converting some or all of the LED's output into visible wavelengths. There are a number of techniques that may be used for wavelength conversion. One method is to deposit a yellow phosphor on a blue LED. This is considered an inexpensive method for producing white light. Blue light produced by an LED excites a phosphor, which then re-emits yellow light. This balanced mixing of yellow and blue lights results in the appearance of white light.
Wavelength conversion may also be accomplished by providing additional phosphors on a blue LED. This is similar to the process involved with yellow phosphors, except that each excited phosphor re-emits a different color. Similarly, the resulting light is combined with the originating blue light to create white light. The resulting light, however, has a richer and broader wavelength spectrum and produces a higher color-quality light, albeit at an increased cost.
Yet another technique to accomplish wavelength conversion is by using an ultraviolet (UV) LED coated with doped phosphors which, upon excitation, emit light in the red, green and blue wavelengths. The UV light is used to excite the different phosphors, which are doped at measured amounts. The colors are mixed resulting in a white light with the richest and broadest wavelength spectrum.
Another technique for wavelength conversion uses a thin layer of nanocrystal particles, called quantum dots, containing 33 or 34 pairs of atoms, primarily cadmium and selenium, which are coated on top of a blue LED. The blue light excites the quantum dots, resulting in a white light with a wavelength spectrum similar to UV LEDs.
Color mixing involves utilizing multiple colored LEDs in a lamp and adjusting the intensity of each LED to produce white light. For example, the lamp may contain a minimum of two LEDs (blue and yellow), but can also have three (red, blue, and green) or four (red, blue, green, and yellow). As no phosphors are used, there is no energy lost in the conversion process, thereby exhibiting the potential for higher efficiency. The intensity of the LEDs are configured such that the combination of the emitted light results in white light.
Wavelength conversion provides benefits versus color mixing. A SSL device contains many LEDs placed close together in a lamp to amplify their illuminating effects. This is because an individual LED produces only a limited amount of light, thereby limiting its effectiveness as a replacement light source. In the case where white LEDs are utilized in SSL, this is a relatively simple task, as all LEDs are of the same color and can be arranged in any fashion. When using the color-mixing method, however, it is more difficult to generate equivalent brightness when compared to using white LEDs in a similar lamp size. Furthermore, degradation of different LEDs at various times in a color-mixed lamp can lead to an uneven color output. Because of the inherent benefits and greater number of applications for white LED based SSL, most designs focus on utilizing them exclusively.
Currently, there is no SSL available that can be offered as a true replacement for incandescent or fluorescent lamps, even though several manufacturers have gone forward with the introduction of such products. White LEDs produced today are too expensive to be considered affordable, and the lumens produced by the LEDs today are not as bright as traditional lighting. Based on research conducted by the United States Department of Energy (DOE) and the Optoelectronics Industry Development Association (OIDA), it is expected that by the year 2025, SSL will be the preferred method of illumination in homes and offices.
What is apparent to the end user is the low color rendering index (CRI) of current LEDs. The CRI is widely used to measure how accurately a lighting source renders the color of objects. For example, sunlight and incandescent lamps have a CRI of 100, while fluorescent lamps generally have a CRI>75. The current generation of LEDs, which employs mostly blue LED chip and yellow phosphor, has a CRI of about 70, which is much too low for widespread use in lighting, particularly indoor lighting applications. In order for SSL to effectively replace incandescent lamps, more research must be done on developing alternatives to the techniques currently used that address these concerns.
There are several advantages to the use of the nano-silicon converter in a white LED. Silicon nanoparticles play a dual role of UV blockers and down converters of the UV radiation emitted by the LED. Silicon nanoparticles are highly absorbant of the UV with a quantum conversion larger than 50%. In fact, silicon nanoparticles may act as a total UV filter, resulting in a safe light source. The silicon nanoparticles stay cool because they convert the UV radiation to visible light. The silicon nanoparticles are highly photostable under UV excitation giving a long safe working lifetime.
Further, a film comprised of silicon nanoparticles acts an excellent antireflection coating preventing light from going back into the LED housing causing damage due to heating or direct interaction. The silicon nanoparticle film is transparent in the visible allowing the visible light to go through.
The nanoparticles within each color group are identical, allowing the formation of high optical quality films of closely-packed nanoparticles (solid density). This is beneficial because the emission, transmission and losses of wavelength converter depends sensitively on thickness uniformity and composition of the converter on the chip.
The nanoparticles can be functionalized (doped) to shift their luminescence under the same UV source. Producing a Si—C termination on the particles, for example, shift the spectrum to the silicon carbide emission. This may provide means to improve on filling the white spectrum to achieve a high CRI ratio in the upper nineties.